Recovery boilers are well known in the pulp and paper industry as a means to recover spent cooking chemicals and the associated heating value to produce steam for process use or power generation. The spent cooking chemicals are recycled after being used to dissolve wood chips to liberate fibers for papermaking. The fibers are separated from the chemical bath, which contains a high concentration of organic material that can be burned in the recovery boiler. The “spent” cooking chemicals are recovered from the chemical bath through the combustion process. In order to recover the spent chemicals and burn the organic matter, much of the water is evaporated from the chemical stream, with the resultant forming concentrated “black liquor” with upwards of 75% solids content (organic and inorganic materials). This black liquor is sprayed into the boiler in an atomizing fashion forming droplets that dry and go through several processes, and expel flammable gasses and char material. To activate some of the cooking chemicals, they are chemically reduced, which requires high heat. Since the total cooking chemicals are inorganic, almost all fall to the floor of the boiler in the form of molten smelt that flows out of the bottom of the recovery boiler to be dissolved, processed and reused. The finer points of recovery boiler design and operation are described in detail in many patents, several of which are cited below. The black liquor is sprayed into the furnace by one or more injection nozzles at an elevation of from 4 to 10 meters or more. Combustion air enters the boiler at several levels via port openings arranged around the perimeter of the boiler, some levels above and some below the liquor spray. The interaction of the combustion air and flammable materials inside the boiler is crucial for the boiler to perform well. In particular, improving the mixing of the air and fuel improves the combustion and many dependent process variables. Being of finite size and heavily loaded, many recovery boilers are at the limit of their ability to process black liquor while using outdated combustion air systems. The combustion air system consists of all of the design parameters and components required to introduce combustion air into the boiler. This includes fans, air heaters, ducting, dampers, port cleaners, instrumentation, controls, actuators, and the size and arrangement of the port openings themselves. The port openings are the openings in the walls of the furnace through which the combustion air enters. The present invention is focused on an improved arrangement of combustion air ports for a recovery boiler.
Typical recovery boilers consist of a floor and walls constructed from heavy steel tubing, welded together forming walls with the tubes running vertically. The walls and floor form a large box that contains the combustion. The tubes are filled with water that circulates through the floor and walls and absorbs heat from the combustion in the boiler. The water eventually flows upward to the convective heat transfer sections located at the top of the boiler. These include the screen tubes, superheater and generating bank. Combustion air is typically injected into the furnace at a variety of levels, with the primary and secondary levels located below the liquor spray, and the tertiary and higher levels located above the liquor spray. Some boilers have combustion air introduced at or very close to the liquor spray level. There may be from one to over ten different levels of combustion air. Many arrangements of combustion air systems are described in the literature and patents, some of the more pertinent examples being U.S. Pat. No. 5,121,700 (Blackwell et al.), U.S. Pat. No. 5,305,698 (Blackwell, et al.), U.S. Pat. No. 5,724,895 (Uppstu), and U.S. Pat. No. 6,302,039 (McCallum et al.). Many of these concepts have been tried on operating boilers and have yielded varying degrees of success. Until recently it has not been possible to test a combustion air system concept thoroughly. Prior to the advent of advanced computational fluid dynamic (CFD) modeling, engineers had to rely on experience, similitude, and mathematical models to predict the performance of a combustion air system design. CFD modeling techniques and software, combined with high performance computers, now permits the accurate, comprehensive, and economical testing of combustion air systems, and the comparison of many different designs. While some CFD modeling may have been used in the development of the above patents, it was not of the sophistication that is currently available. Extensive “estate of the art” CFD modeling was used to develop the present invention and to test various combustion air systems including many of those cited in the above references. The Combustion air system described in this application has been shown to outperform the older combustion air system designs in a variety of ways.
The invention described here is mainly concerned with the arrangement of the combustion air port openings through the boiler walls. The arrangements of the fans, heaters, ducting, etc., are typically employed according to common engineering practice, with the exceptions detailed below. It has been revealed by experience and CFD modeling that the air jets emitted from the combustion air ports must be at least partially interlaced in order to be effective at mixing the fuel and air while limiting the carryover associated with high vertical gas velocities in the boiler. Carryover is the particulate matter that is a by-product (or portion of) the black liquor sprayed into the boiler that is entrained in the vertical gas flow. The combustion air mainly flows upward in the boiler carrying particles to the convective heat transfer surfaces where it can eventually plug the entire boiler. High vertical velocities and poor mixing also carry high temperatures into the upper boiler because combustion is delayed, and transport times are faster. The combination of high carryover and high temperatures causes rapid fouling in the upper furnace.
Many older combustion air systems employ air ports arranged in several levels. A “level” consists of all those combustion air ports arranged at about the same elevation on the boiler and excludes burner ports, camera ports, and etc. The primary level typically consists of one or two horizontal rows of air ports on all four sides of the boiler. The primary level is the lowest level in the boiler and may supply up to 50% or more of the total required combustion air. Above the primary level but below the liquor spray is the secondary level or levels. Common practice is to have a single secondary level but zero to four or more levels have been tried. The secondary level may supply up to 50% of the total combustion air. Above the liquor spray is the tertiary level. The tertiary most typically consists of a single level but may have 6 or more levels. The tertiary may supply up to 50% of the total combustion air, but 20% is more typical. Some boilers are fitted with a quaternary level above the tertiary, but the delineation is often merely semantics. For the sake of this discussion, all levels above the liquor spray will be referred to as tertiary air, unless that level is fed with something other than combustion air (e.g. re-circulated flue gas or dilute non-condensable gasses).
An interesting development in recovery boiler air systems is described in the Uppstu patent, U.S. Pat. No. 5,724,895. This patent details a “vertical” air system with many secondary and tertiary levels. This “vertical” system has many air levels, but practice has shown that it is virtually impossible to use all these levels and openings because, to do such would mean that the airflow mass at each opening would be too little to have any influence on the mixing of air and fuel. The energy supplied with the combustion air system supplies the mixing energy for combustion, and if the airflow from an air jet is too weak, then the mixing is subsequently weak. This air system was designed to limit NOx emissions in Scandinavia and is successful but limited in other ways and expensive to retrofit. For example, the production of NOx in a recovery boiler is a function of the combustion temperature. Higher temperatures form more oxides of Nitrogen than cooler combustion temperatures. Therefore the vertical air system is designed to “stage” the combustion to keep the peak flame temperatures down. While this helps to control NOx formation it may not reduce total reduced sulfur (TRS), and may not improve reduction efficiency, heat transfer, or char bed control, and may delay final combustion until high in the boiler where higher gas temperatures can be a problem as described earlier. The vertical air system is expensive to retrofit because more than seven levels of port openings and ducting must be installed. Where the vertical air system is successful is in creating vertical mixing zones in the furnace that improve the mixing and combustion to the extent that combustion air is limited (i.e. staged), but not all openings nor levels are simultaneously in use. While reducing NOx emissions is valuable, the overall benefit of the vertical air system is limited and the implementation is expensive. The invention described herein is an improved combustion air system that controls NOx emissions while also improving reduction efficiency, improving heat transfer to the boiler walls, improving boiler water circulation, improving char bed control, reducing carryover, reducing gas temperatures in the upper furnace, and reducing TRS and CO emissions, and is economical to implement.